Deep brain stimulation (DBS) and other related procedures involving implantation of electrical stimulation leads within the brain of a patient are increasingly used to treat disorders, such as Parkinson's disease, dystonia, essential tremor, seizure disorders, obesity, depression, restoration of motor control, and other debilitating diseases via electrical stimulation of one or more target sites, including the ventrolateral thalamus, internal segment of globus pallidus, substantia nigra pars reticulate, subthalamic nucleus, or external segment of globus pallidus. DBS has become a prominent treatment option for many disorders, because it is a safe, reversible alternative to lesioning. For example, DBS is the most frequently performed surgical disorder for the treatment of advanced Parkinson's Disease. There have been approximately 30,000 patients world-wide that have undergone DBS surgery. Consequently, there is a large population of patients who will benefit from advances in DBS treatment options.
During DBS procedures, at least one burr hole is meticulously cut through the patient's cranium without puncturing the dura layer that lines the inner surface of the skull or damaging the brain tissue below. A large stereotactic targeting apparatus is then mounted to the patient's cranium, and a cannula is scrupulously positioned towards the target site in the brain. A stimulation lead is then introduced through the cannula, through the burr hole, through the exposed dura layer, and into the parenchyma of the brain, such that one or more electrodes located on the lead are strategically placed at a target site in the brain of the patient. Typically, an imaging device, such as a magnetic resonant imager (MRI), will be used to visualize the lead relative to the target site. Once the lead is properly positioned, the portion of the lead exiting the burr hole is subcutaneously routed underneath the patient's scalp to an implantable pulse generator (IPG) implanted in the patient at a site remote from the burr hole (e.g., the patient's shoulder or chest region). Further details discussing the treatment of diseases using DBS are disclosed in U.S. Pat. Nos. 6,845,267 and 6,950,707, both of which are expressly incorporated herein by reference.
Significantly, it is crucial that proper location and maintenance of the lead position be accomplished in order to continuously achieve efficacious therapy. This is especially so with DBS applications, in which cases, the target site (or sites) that is intended for electrical stimulation is about the size of a pea and is located deep within the patient's brain. Thus, lead displacements of less than a millimeter may have a deleterious effect on the patient's therapy. Therefore, it is important that the electrode(s) of the lead be accurately located at the target site and that such electrode(s) be securely maintained at the target site during and after implantation of the lead.
Brain shift is a prevalent source of significant error in the implantation of stimulation leads. Brain shift may include the movement and deformation of the brain during an operation. For example, when the dura is punctured to access brain tissue during the stimulation lead implantation procedure described above, sub-cranial contents may shift significantly due to cerebrospinal fluid (CSF) leaking out of the burr hole, which causes a change in intracranial pressure. Surgeons rely on pre-operation MRI scans to pinpoint the electrodes' targets and these targets must be hit with millimeter-quality accuracy. Fixed fiducials on the surface of the cranium are used as reference points for the insertions. However, these fiducials cannot take into account the shifting of the sub-cranial contents, and by the time the actual leads are implanted, the MRI guiding the physicians is inaccurate. Over the course of a surgery, comprehensive studies have often recorded average brain displacements approaching or exceeding ten millimeters, which is an unacceptable margin of error. The result is that doctors are forced to use blind guess-and-check methods in order to locate points deep within the brain.
Brain shift is generally caused by settling of the brain, often facilitated by a leakage of CSF and the resulting decrease of intracranial pressure and buoyancy. This could be caused by a failure to seal a burr hole when dura puncture occurs, another such related accident, or may be an innate aspect of some surgical techniques. Brain shift is not always uniform deep within the tissue, adding the complication of deformation. The primary force driving brain shift is invariably gravity, though deformation could potentially be caused by osmotic drugs that change the brain's water concentrations. A study of pre-operation MRIs has shown that brain shift from changes in patient position is insignificant (less than 1 mm).
To address these issues, there are several protocols now being developed to compensate for the discrepancy between the MRI and the shifted brain. The most effective of these involve intra-operative MRIs, ultrasounds, or optical scanners designed to update the MRI over the course of the surgery. MRIs, in particular, provide the most information, but are prohibitively expensive, with unit prices in the millions of dollars, and take far too long per scan to be efficient in the operating room. In addition, not all components of current DBS systems (e.g., the leads) are MRI compatible. Thus, the use of MRI during surgery may not be advantageous. Other methods involve the creation of computer models to predict displacement.
As an alternative to compensating for brain shift, there remains a need for reducing or preventing brain shift in a manner that is medically and financially preferable.